Flexible piezoelectric structures and method of making same

ABSTRACT

A flexible piezoelectric structure and a method of making the structure are disclosed. A piezoelectric film having a relatively high piezoelectric coefficient is attached to a flexible substrate. The piezoelectric film is fabricated on a different substrate and transferred to the flexible substrate by contact.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/258,943, filed on Nov. 6, 2009 and U.S. Provisional Application No.61/259,304, filed on Nov. 9, 2009, which are incorporated by referenceas if fully set forth.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant No.2008*1218103*000 awarded by the Central Intelligence Agency. Thegovernment has certain rights in this invention.

FIELD OF INVENTION

This is directed to flexible piezoelectric structures and a method formaking the structures.

BACKGROUND

Stretchable, biocompatible rubbers may yield novel implantable orwearable energy harvesting systems. As one example, decreasing powerrequirements for mobile electronic devices open the possibility ofcharging such devices by continuously extracting otherwise wasted energyfrom the environment. Such charging could be done with or withoutadditional energy supplied by a battery.

Another attractive possibility is that of utilizing work produced by thehuman body via everyday activities, such as breathing or walking. Theheel strike during walking is a particularly rich source of energy, with67 watts of power available from a brisk walker. Harvesting even 1-5% ofthat power would be sufficient to run many body-worn devices such asmobile phones. Similarly, it has been shown that lung motion bybreathing can generate up to 1 W of power. If this power were harvestedinto charging a pacemaker battery, for example, it may increase the timerequired between battery replacement surgeries for patients.

Crystalline piezoelectric materials are promising materials forelectromechanical energy conversion technologies. These materials becomeelectrically polarized when subjected to a mechanical stress, andconversely experience a strain in response to an applied electric field,the strain being in proportion to the strength of that field.Single-crystal perovskites, such as lead zirconate titanate (PZT), arean exceptionally efficient class of energy conversion materials. Indeed,conversion of mechanical to electrical energy with efficiencies above80% has been demonstrated using PZT piezoelectric cantilevers operatednear resonance.

Epitaxial growth of such crystalline materials depends on the use ofrigid, inorganic host substrates, as well as high temperature depositionprocesses. For example, rf sputtering at 600° C. has been shown to yieldsingle-crystal films of PZT over large areas with excellentcompositional control when deposited on MgO or SrTiO₃ substrates.However, next-generation applications, such as wearable energyharvesting systems, may require the piezoelectric materials to beflexible, lightweight, and biocompatible. The flexible piezoelectricpolymer polyvinylidene difluoride (PVDF) has been used for applicationssuch as shoe-sole power generators and implantable breath harvesting.

SUMMARY

A flexible piezoelectric structure and a method of making the structureare disclosed. A piezoelectric film having a relatively highpiezoelectric coefficient is attached to a flexible substrate. Thepiezoelectric film is fabricated on a different substrate andtransferred to the flexible substrate by contact.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of a flexible piezoelectric structure.

FIGS. 2A-2F show an embodiment of a method of fabricating a flexiblepiezoelectric structure.

FIGS. 3A and 3B show respectively one step in a method of fabricating aflexible piezoelectric structure and the resulting structure component.

FIG. 4 shows an embodiment of a flexible piezoelectric structure with analternate electrode configuration.

FIG. 5 shows an embodiment of a flexible piezoelectric structure withanother alternate electrode configuration.

DETAILED DESCRIPTION

Piezoelectric polymers, such as PVDF, suffer a number of drawbacks. Thepiezoelectric coefficient d₃₃ for the organic polymer PVDF, 26picoCoulombs/Newton (pC/N), is almost ten times smaller than forinorganic PZT, which may exceed 250 pC/N. PZT-PVDF composites can showhigh performance, but are known to degrade rapidly in air. Piezoelectriccrystalline materials, such as PZT, represent a particularly interestingsubset of smart materials which function as sensors, actuators, andenergy converters. Yet, the crystallization of these materials generallyrequires relatively high temperatures for maximally efficientperformance, rendering them incompatible with temperature-sensitiveplastics and rubbers.

The difficulties described above may be overcome with a device structureand a process for fabricating that structure disclosed here. The processis scalable in area for transferring crystalline piezoelectric filmswith relatively high piezoelectric coefficients, such as PZT, from hostsubstrates onto flexible rubbers over macroscopic areas. The resultingstructure includes a material of relatively high piezoelectriccoefficient on a flexible substrate. Such structures may enable a hostof novel applications. The disclosed piezoelectric structure isflexible, light in weight, and may be made biocompatible. It maytherefore be suitable for providing electrical energy for wearableand/or implantable devices. The described structure has relativelyhigher mechanical-electric coupling, and is therefore more efficient atenergy conversion than polymer type piezoelectric materials such as PVDFor composite type piezoelectric materials like a PZT/polymer composite.

The method and device structures described here may be used for makingwearable or implantable energy harvesting devices such as shoe-solepower generators or implantable devices harvesting energy from motionsof breathing. The device structures may replace batteries or augmentthem with a portable charging source. The device structures may also belaminated against vibrating machines to harvest otherwise wasted energyby mechanical motors or internal combustion engines.

The method and device structures described here may provide continuousscavenging of energy from the environment in which they reside.Advantages of this type of energy harvester may include a high degree ofportability, flexibility, stretchability, and low weight. Otheradvantages may include biocompatibility and higher resistance todegradation due to air, water, heat, and other environmental factors.Still other advantages include relative ease and low cost of fabricationcompared to bulk piezoelectric crystals.

The structure described here may also be used in an opposite fashion, inwhich the structure undergoes mechanical motion in response to appliedelectrical energy. The structure could thus be used to convertelectrical energy to mechanical energy. For example, the structure couldbe part of a wing that flaps in response to an electrical signal,thereby enabling an object to fly.

The method and device structures disclosed here include at least onethin film of piezoelectric material on a flexible substrate. Thestructure may include a patterned piezoelectric film on a flexiblesubstrate. The piezoelectric film may be patterned as a plurality ofribbons or belts having a non-circular cross-section. Such patterns willbe referred to hereafter as ribbons. Such ribbons are distinct fromother structures in which the piezoelectric material is in the form of“wires” or “fibers” having a circular cross section.

FIG. 1 shows an embodiment of a device structure 100, not intended to belimiting. Device structure 100 includes a patterned piezoelectric film110 attached to a first flexible substrate 105. In this embodimentpiezoelectric film 110 is patterned as a plurality of ribbons on asurface of first flexible substrate 105. These ribbons 110 have anon-circular cross-section and differ from wires or fibers as explainedabove.

Piezoelectric film 110 may contain a non-polymeric material, anon-crystalline material, a partially crystalline material, apolycrystalline material, or a crystalline material. Piezoelectric film110 may have a piezoelectric coefficient significantly greater than thatof PVDF and of other piezoelectric organic polymers. Piezoelectric film110 may contain an inorganic material such as a perovskite. Suitablematerials for use as piezoelectric film 110 include, but are not limitedto, PZT, ZnO, Quartz, LiNbO₃, SrTiO₃, BaTiO₃, lead zirconate, or leadtitanate, in any combination.

Still referring to the embodiment shown in FIG. 1, a second flexiblesubstrate 115 is shown. Two electrodes 120A and 120B are attached to aside of second flexible substrate 115 facing a surface of first flexiblesubstrate 105 upon which patterned piezoelectric film 110 is situated.First flexible substrate 105 and second flexible substrate 115 may bebrought together and bonded to each other, as indicated by the arrows atthe right of FIG. 1, forming a single integral structure with firstelectrode 120A and second electrode 120B in contact with patternedpiezoelectric film 110. First electrode 120A and second electrode 120Bprovide means of making electrical contact with patterned piezoelectricfilm 110 so that electrical energy produced by the flexing of patternedpiezoelectric film 110 may be collected. First electrode 120A and secondelectrode 120B may be made by depositing a conducting film, such as ametal film, on second flexible substrate 115 and patterning theconducting film using known techniques. In the embodiment shown in FIG.1, first and second electrodes 120A and 120B are shown as aninterdigitated pair, with fingers running perpendicular to the ribbons.Alternative configurations of electrodes can be used, depending, forexample, on the type of piezoelectric response exhibited by patternedpiezoelectric film 110. In one alternative, two electrodes may makecontact respectively on opposite sides of patterned piezoelectric film110 and/or electrode fingers may run predominantly parallel to theribbons. FIG. 5 shows a cross section of one such possibleconfiguration. In this configuration piezoelectric ribbons 215 aresandwiched between electrode 520A and electrode 520B. Furthermore, bothelectrodes 520A and 520B run parallel to piezoelectric ribbons 215.Electrode 520A is in contact with flexible substrate 220. Anotheralternative electrode configuration is shown in FIG. 4. In thisalternative first electrode 420A and second electrode 420B are both incontact with one side of patterned piezoelectric film 410 and aresituated between patterned piezoelectric film 410 and flexible substrate415.

Patterned piezoelectric film 110 may be completely encapsulated by, forexample, first flexible substrate 105 and second flexible substrate 115in FIG. 1. In this way, it may be possible to make a structure which isfully biocompatible even when piezoelectric film 110 contains a toxicmaterial such as lead, as is the case with PZT.

In another alternative, a second patterned piezoelectric material (notshown) may be fabricated either alongside or stacked over patternedpiezoelectric film 110. With multiple fabrications of patternedpiezoelectric material alongside each other, structures of arbitrarylarge area can be produced, as explained below.

Suitable materials for first flexible substrate 105 and second flexiblesubstrate 115 include, but are not limited to, polydimethylsiloxane(PDMS), polyethylene terephthalate (PET), polyether ether ketone (PEEK),a polyimide such as Kapton®, or a polyester such as Mylar®, all in anycombination.

FIGS. 2A-2F show an embodiment of a process for fabricating a flexiblepiezoelectric structure. FIGS. 2A-2F and the following descriptions donot include some steps, such as cleaning and heat treatments, which areknown to a person of ordinary skill in the art. The followingdescription of a process embodiment is an example and is not to beconstrued as limiting.

Starting in FIG. 2A a patterned sacrificial layer 210 is formed on asubstrate 205. One suitable material for substrate 205 is MgO. Patternedsacrificial layer 210 may be formed using photoresist. The photoresistmay be spin-coated onto substrate 205 and patterned usingphotolithography. In this embodiment the sacrificial layer is given anegative pattern—that is, the pattern of the sacrificial layer isdesigned to be the negative complement of a desired final pattern of apiezoelectric film.

As shown in FIG. 2B, a piezoelectric film 215 is formed over patternedsacrificial layer 210. Piezoelectric film 215 may be formed by filmdeposition techniques including, but not limited to, sputtering,including rf sputtering; hydrothermal deposition; chemical vapordeposition (CVD), including metal-organic CVD; sol-gel deposition; laserablation; pulsed laser deposition; or molecular beam epitaxy. Filmdeposition may be followed by annealing of the film which may increasethe film piezoelectric coefficients. Piezoelectric film 215 may be anon-polymeric, non-crystalline, partially crystalline, polycrystalline,or a crystalline material. Piezoelectric film 215 may have apiezoelectric coefficient significantly greater than that of PVDF and ofother piezoelectric organic polymers. Piezoelectric film 215 may be aninorganic material such as a perovskite. Suitable materials for use aspiezoelectric film 215 include, but are not limited to, PZT, ZnO,Quartz, LiNbO₃, SrTiO₃, BaTiO₃, lead zirconate, or lead titanate, in anycombination. Piezoelectric film 215 may have a thickness ranging from afew hundred nanometers to several micrometers.

As shown in FIG. 2C, patterned sacrificial layer 210 may then beremoved, taking with it undesired portions of piezoelectric film 215.Removal of patterned sacrificial layer 210 may be done in a liquidsolution. This is often referred to as a lift-off process.

As shown in FIG. 2D, substrate 205 is undercut in order to reduce anarea of contact between substrate 205 and piezoelectric film 215. Thismay be accomplished, for example, using a wet etch to partially dissolvesubstrate 205. This will have the effect of reducing the overallthickness of substrate 205 while undercutting, as shown by comparingFIGS. 2C and 2D.

As shown in FIG. 2E a flexible substrate 220 may be brought intoconformal contact with patterned piezoelectric film 215. Suitablematerials for flexible substrate 220 include, but are not limited to,polydimethylsiloxane (PDMS), polyethylene terephthalate (PET), polyetherether ketone (PEEK), a polyimide such as Kapton®, or a polyester such asMylar® in any combination.

Flexible substrate 220 may then be separated from substrate 205,resulting in the structure shown in FIG. 2F. If adhesive forces betweenflexible substrate 220 and patterned piezoelectric film 215 exceedadhesive forces between patterned piezoelectric film 215 and substrate205, then patterned piezoelectric film 215 may be transferred intact toflexible substrate 220. The undercutting of substrate 205 shown in FIG.2D enhances the likelihood of a successful transfer.

In the embodiment shown in FIGS. 2A-2F piezoelectric film 215 is shownpatterned into ribbons with non-circular cross-sections, as distinctfrom wires or fibers having circular cross-sections, as described above.In an alternative embodiment, the piezoelectric film may be unpatterned.An unpatterned film may produce greater electrical power for a givenmechanical stress than a patterned film, at least because theunpatterned film has greater volume.

The process of FIGS. 2A-2F is scalable to any area. The steps of FIGS.2A-2E may be repeated with the same flexible substrate 220 to transfer asecond patterned piezoelectric film to a region of flexible substrate220 alongside first patterned piezoelectric film 215. In similar manner,an arbitrary area of flexible substrate 220 may be covered withpatterned piezoelectric film.

In a similar manner, multiple patterned piezoelectric films may bestacked one on top of another on a single flexible substrate 220 byrepeating the process of FIGS. 2A-2F.

Once at least one patterned piezoelectric film 215 has been transferredto flexible substrate 205, electrodes may be attached to patternedpiezoelectric film 215, resulting in a final structure such as thatshown in FIG. 1 and discussed above.

FIG. 3A shows a perspective view of the transfer of patternedpiezoelectric film 215 to flexible substrate 220 and FIG. 3B shows aperspective view of flexible substrate 220 with completely transferredpatterned piezoelectric film 215. All guide numbers correspond tofeatures shown in FIG. 2.

Obtaining highly crystalline piezoelectric materials such as PZT may beimportant to performance as studies have indicated an inherent reductionin the piezoelectric coefficient of thin PZT films due to internaldefects. Likewise, in the case of PZT, generating ribbons with astoichiometric composition near the morphotropic phase boundary of PZT(Pb[Zr_(x)Ti_(1-x)]O₃ with x=0.52) is needed for maximum piezoelectricresponse and poling efficiency. Poling refers to processes in which theelectric polarization in a region of a piezoelectric material ispermanently altered by the application of an electric field for alimited time.

Example structures were fabricated using PZT films patterned in multipleribbons similar to the example shown in FIG. 3B. Compositional andstructural characterization of the PZT film from which the ribbons werepatterned was done. X-ray diffraction (XRD) and scanning electronmicroscopy (SEM) were used to characterize the structure of the PZTfilm. XRD data shows clear peaks corresponding to the perovskitestructure (100) and (200) faces, indicating a tendency toward epitaxialgrowth with a c-axis perpendicular to the film surface, while theperovskite (111) and pyrochlore peaks are relatively minor. Indeed, theSEM image shows no obvious surface texture. Energy dispersivespectroscopy (EDS) was used to characterize the composition of the PZTfilm. A comparison of the EDS curves for an annealed sample against astandard sample (52/48, Zr/Ti) showed peak intensities that were nearlyidentical for each element, indicating insignificant lead and zirconiumloss during growth and post-annealing.

An important parameter for characterizing fundamental piezoelectricperformance is d, the piezoelectric charge constant as discussed inIEEE/ANSI 176 IEEE Standard on Piezoelectricity (1987), which isincorporated herein by reference. This value represents the polarizationgenerated per unit of mechanical stress applied to a piezoelectricmaterial, or, conversely, the mechanical strain experienced by apiezoelectric material per unit of electric field applied. Thepiezoelectric charge coefficient is a tensor, with components d_(ij),where i indicates the direction of polarization generated in thematerial when the electric field is zero (or the direction of theapplied field strength), and j is the direction of the applied stress(or the induced strain). Obtaining high d values is essential for energyharvesting applications, as the energy conversion efficiency(electromechanical coupling factor, k) of piezoelectric materials scalesproportionally with d.

One of the piezoelectric constants is d₃₁, in the transverse operationmode. This mode has been used in applications such as energy generationfrom piezoelectric shoe implants. Using a wafer flexure method, d₃₁ wasdetermined to be 49 pC/N for an as-annealed sample. This spontaneouspolarization of the film confirms the tendency of the c-axis of the filmto be the out of plane direction. Significantly, this value increased tod₃₁=79 pC/N when the sample was poled at −100 kV/cm for 14 hours. Thesevalues are in agreement with the best reported data for PZT films grownby sputtering and are a factor of 3-4 times higher than typical valuesfor PVDF (d₃₁,PVDF=20-25 pC/N).

Another useful quantity for characterizing the performance of apiezoelectric material is d₃₃, the induced polarization per unit stressapplied in the longitudinal (poling) direction. To characterize theperformance of PZT ribbons, d₃₃ measurements were conducted before andafter transfer printing to a flexible substrate, as described above. APZT film with a Pt bottom contact electrode on an MgO substrate wasanalyzed using Piezoresponse Force Microscopy (PFM). A value of d₃₃=82pC/N was measured. The film was subsequently continuously scanned with a10 V DC bias voltage (100 kV/cm) over a small area (100 n m²) for 30 minto pole the sample; d₃₃ was found to increase to as high as 173 pC/Nafter poling. This thin film value is comparable to values fororganometallic chemical vapor deposited PZT films.

Direct d₃₃ measurements were performed on PZT ribbons after transfer toPDMS. To apply an AC bias voltage, the PDMS was doped with 13% wt carbonblack to render it conducting. For the as-transferred sample, d₃₃ wasdetermined to be 40 pC/N, while with poling was found to yield d₃₃=79pC/N. This value represents a 3-fold improvement over previouslymeasured values for ZnO nanobelts on rigid substrates(d_(33,ZnO Nanobelt)=27 pC/N).

The electromechanical energy conversion metrics disclosed here are amongthe highest reported on a flexible medium. The excellent performance ofthe piezo-ribbon assemblies coupled with stretchable, biocompatiblerubber may enable a host of novel applications.

While several embodiments have been described and illustrated herein,those of ordinary skill in the art will readily envision a variety ofother means and/or structures for performing the functions and/orobtaining the results and/or one or more of the advantages describedherein, and each of such variations and/or modifications is deemed to bewithin the scope of the present embodiments. More generally, thoseskilled in the art will readily appreciate that all parameters,dimensions, materials, and configurations described herein are meant tobe exemplary and that the actual parameters, dimensions, materials,and/or configurations will depend upon the specific application orapplications for which the teachings herein is/are used. Those skilledin the art will recognize, or be able to ascertain using no more thanroutine experimentation, many equivalents to the specific embodimentsdescribed herein. It is, therefore, to be understood that the foregoingembodiments are presented by way of example only and that, within thescope of the appended claims and equivalents thereto, the embodimentsdisclosed may be practiced otherwise than as specifically described andclaimed. The present embodiments are directed to each individualfeature, system, article, material, kit, and/or method described herein.In addition, any combination of two or more such features, systems,articles, materials, kits, and/or methods, if such features, systems,articles, materials, kits, and/or methods are not mutually inconsistent,is included within the scope of the present embodiments.

1. A flexible piezoelectric structure, comprising: a first flexiblesubstrate; and a first piezoelectric film attached to the flexiblesubstrate.
 2. The flexible piezoelectric structure of claim 1, whereinthe first piezoelectric film is patterned.
 3. The flexible piezoelectricstructure of claim 2, wherein the first piezoelectric film is patternedas ribbons having a non-circular cross-section.
 4. The flexiblesubstrate of claim 1, wherein the first piezoelectric film comprises aninorganic material.
 5. The flexible piezoelectric structure of claim 1,wherein the first piezoelectric film comprises at least one of anon-polymeric material, a non-crystalline material, a partiallycrystalline material, a polycrystalline material, or a crystallinematerial.
 6. The flexible piezoelectric structure of claim 1, whereinthe first piezoelectric film has a piezoelectric coefficient exceeding26 picoCoulombs/Newton.
 7. The flexible piezoelectric structure of claim1, wherein the first piezoelectric film comprises at least one of PZT,ZnO, Quartz, LiNbO₃, SrTiO₃, BaTiO₃, lead zirconate, or lead titanate.8. The flexible piezoelectric structure of claim 1, further comprising asecond flexible substrate, wherein the first piezoelectric film isencapsulated by the first and second flexible substrates.
 9. Theflexible piezoelectric structure of claim 1 further comprising first andsecond electrodes in contact with the first piezoelectric film.
 10. Theflexible piezoelectric structure of claim 9, wherein the first andsecond electrodes are both in contact with one side of the firstpiezoelectric film.
 11. The flexible piezoelectric structure of claim 9,wherein the first and second electrodes are respectively in contact withopposite sides of the first piezoelectric film.
 12. The flexiblepiezoelectric structure of claim 3 further comprising electrode fingersrunning parallel to the ribbons.
 13. The flexible piezoelectricstructure of claim 3 further comprising electrode fingers runningperpendicular to the ribbons.
 14. The flexible piezoelectric structureof claim 1, wherein the flexible substrate comprises at least one of:polydimethylsiloxane (PDMS), polyethylene terephthalate (PET), polyetherether ketone (PEEK), a polyimide, or a polyester.
 15. The flexiblepiezoelectric structure of claim 1, further comprising a secondpiezoelectric film stacked over the first piezoelectric film.
 16. Amethod for fabricating a flexible piezoelectric structure, comprising:depositing a first piezoelectric film on a first substrate; contacting aflexible substrate with the first piezoelectric film; and separating thefirst piezoelectric film and flexible substrate from the firstsubstrate, the first piezoelectric film remaining attached to theflexible substrate.
 17. The method of claim 16, further comprisingpatterning the first piezoelectric film.
 18. The method of claim 17,wherein the patterning comprises patterning the first piezoelectric filminto ribbons having a non-circular cross-section.
 19. The method ofclaim 16, wherein depositing comprises at least one of sputtering,radio-frequency sputtering, hydrothermal deposition, chemical vapordeposition (CVD), metal-organic CVD, sol-gel deposition, laser ablation,pulsed laser deposition, or molecular beam epitaxy.
 20. The method ofclaim 16, wherein the first piezoelectric film comprises an inorganicmaterial.
 21. The method of claim 16, wherein the first piezoelectricfilm comprises at least one of a non-polymeric material, anon-crystalline material, a partially crystalline material, apolycrystalline material, or a crystalline material.
 22. The method ofclaim 16, wherein the first piezoelectric film comprises a materialhaving a piezoelectric coefficient exceeding 26 picoCoulombs/Newton. 23.The method of claim 16, wherein the first piezoelectric film comprisesat least one of: PZT, ZnO, Quartz, LiNbO₃, SrTiO₃, BaTiO₃, leadzirconate, or lead titanate.
 24. The method of claim 16, furthercomprising encapsulating the first piezoelectric film with a secondflexible substrate.
 25. The method of claim 16, further comprisingfabricating first and second electrodes in contact with the firstpiezoelectric film.
 26. The method of claim 16, wherein the first andsecond electrodes are both fabricated in contact with one side of thefirst piezoelectric film.
 27. The method of claim 16, wherein the firstand second electrodes are respectively fabricated in contact withopposite sides of the first piezoelectric film.
 28. The method of claim18, further comprising fabricating electrode fingers running parallel tothe ribbons.
 29. The method of claim 18, further comprising fabricatingelectrode fingers running perpendicular to the ribbons.
 30. The methodof claim 16, wherein the flexible substrate comprises at least one of:polydimethylsiloxane (PDMS), polyethylene terephthalate (PET), polyetherether ketone (PEEK), a polyimide, or a polyester.
 31. The method ofclaim 16, further comprising repeating the depositing, contacting andseparating to dispose at least one additional piezoelectric film stackedover the first piezoelectric film.